This invention relates to insulators, and more particularly, to cylindrical insulators, such as insulating members of vacuum circuit-breakers, insulation operating rods, or the like which are disposed in dirty or wet environments, each insulator comprising insulation belts or bands to increase insulation capacity and creeping distance.
Recently, design emphasis has been on compact electric installations which are designed within the limitations of conventional insulation techniques. Vacuum circuit-breakers are typical examples. They have been largely reduced in shape and have been widely used in a piled arrangement in multi-stages in a switchboard. In many instances they are used in adverse circumstances under dirty or wet conditions.
Generally, an insulating vessel of a vacuum circuit-breaker, i.e., a vacuum switch, is required to possess a high insulation ability and a high gas-tight characteristic in addition to being compact. For these requirements, the vacuum switch is generally made of glass or ceramics and has a cylindrical configuration. However, since the insulating vessel of such a vacuum switch has a creeping surface insulation distance less than that of a conventional corrugated insulator, voltage stress on the creeping surface will become high and the insulation characteristics will be degraded in a case where the vacuum switch is disposed in contaminated or wet environments. Particularly, during a breaking time of a capacitor circuit, an interelectrode voltage is duplicated, so that a flashover phenomenon may easily occur on the surface of the insulating vessel of the vacuum circuit-breaker causing it to fail in its circuit-breaking function.
At a time of inspection or maintenance of a vacuum switch which is contained in a casing such as that disposed on a pole, the voltage at the outside of the casing is inspected by a voltage detector to confirm the condition of contact opening of the switch, and in this case, when an insulation condition on the outer surface of the insulating vessel is degraded by dirt, dust, or moisture, the detector may erroneously detect the voltage even if the contact of the switch is normally open. In addition, with insulators made of organic materials such as insulation operating rods of switching mechanisms, a tracking phenomenon, which finally results in insulation breakdown in an adverse case, may be developed by creeping discharge when the outer surfaces of the insulators are under dirty or wet conditions.
From further point of view, recently there has been provided a vacuum switch which has an outer surface coated with an insulating film such as a resin film, the thus coated resin may easily be dried and the water repellent characteristic of the resin will be degraded as time elapses because of adhered dust or the like.